In vehicle motor applications, Roots-type blower superchargers are used for transferring volumes of air into the combustion chambers of an engine. The transferred volumes of air are greater than the displacement of the engine, thereby increasing the air pressure within the combustion chambers which results in greater engine output power.
A Roots-type blower is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing, lobed rotors provided in overlapping rotor chambers. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to an outlet side.
Modern Roots-type blowers typically have twisted lobes, i.e., the rotor lobes define a helix angle greater than zero relative to the axial direction of the rotor. Another significant parameter in a Roots-type blower is the twist angle of each lobe, i.e., the angular displacement in degrees when travelling along a lobe from one end of the rotor to the other end of the rotor.
A long-known problem with Roots-type blowers is that they generate high levels of pulsation noise. As disclosed in US 2006/0263230 A1, the noise can be reduced by increasing the helix angle of the lobes. A large helix angle results in many “blowholes” being formed in connection with meshing of the lobes as the rotors rotate. The blowholes permit communication between adjacent pockets of fluid, which allows for pressure equalization prior to opening the outlet port. Pressure equalization is known to reduce air turbulence (pulsation) and hence pulsation noise.
However, even with many blowholes a Roots-type blower still may produce a considerable amount of noise. Especially, a Roots-type blower may cause a lot of nuisance in a vehicle if run hard at low engine speeds, as the engine at low speeds does not produce sufficient noise to drown the noise from the Roots-type blower.
There is thus a need for an improved Roots-type blower at least partly removing the above mentioned disadvantage.